Conventionally, cadmium zinc tellurium (CZT) and cadmium tellurium (CdTe) crystals can be used to detect the presence of radiation at room temperature. CZT and CdTe crystals accomplish this by directly converting X-ray or y-ray photons into electrons and holes. These electrons and holes are influenced by the application of an applied bias voltage and generate an electric current, which can be measured and recorded. CZT and CdTe crystals are unique semiconductors, because they operate at room temperature and can process up to about 10 million photons/second/mm2. Thus, CZT and CdTe are promising room-temperature nuclear radiation detector materials. For many years, CZT single crystals have captured significant interest due to their wide band gap energy, large atomic number, high resistivity, excellent transport properties for electrons, and good chemical stability. These properties make it suitable for production of room-temperature X-ray and y-ray detectors and imaging arrays.
CdTe and CZT crystals are conventionally grown by melting CdTe and CZT and allowing the melt to crystallize. Traveling heater, horizontal Bridgman, vertical Bridgman, and high pressure Bridgman methods have been conventionally used to grow CdTe and CZT crystals from the melt or from the vapor phase. In the past, crystals grown by such processes tend to suffer from high cost and small crystal size. In addition, the crystals produced by these melt and vapor phase processes tend to have poor electrical and physical characteristics that greatly limit their sensitivity and application as economical radiation detectors. There is a long felt need for a technique for growing high purity, low-cost single CdTe and CZT crystals of a size suitable for high sensitivity detection at high resolution.
CZT single crystals can now be grown up to hundreds of cubic centimeters. Several types of device configurations that rely only on the transport of electrons have been proposed to fully utilize all the advantages of such large-volume CZT crystals. However, big CZT detectors have not achieved their anticipated performance, because the energy resolution was found to degrade with device thickness, particularly for long-drift-length devices (e.g., >5-10 mm). The typical energy resolution measured for 10-15-mm thick detectors is 3-5% on average and ˜2% for the best devices. One of the reasons for this degradation is the effects caused by tellurium (Te) inclusions in the bulk of the crystals. For example, the presence of these inclusions can be revealed by using infrared (IR) transmission microscopy or high spatial resolution X-ray mapping techniques.
Large crystals are more likely to absorb stray gamma rays or x-rays created by radioactive decays leading to an electronic signal upon absorption. A disadvantage of large crystals is that they usually contain more defects that degrade the material properties and reduce the quality of the signal. One of the most common defects in CZT and CdTe crystals are tellurium inclusions. As background, the term “inclusion” is generally used to describe any volume of material trapped inside a crystal during its formation. Typical inclusions can be 1-30 micro m in diameter, are faceted in many cases, can possess voids within their volumes, and have been identified as containing primarily solid tellurium. For example, FIG. 1 illustrates Te inclusions (with average diameter of ˜30 micro m) seen as dark spots in an IR image taken for a typical CZT sample.
It is well known that CZT (and CdTe) crystals melt congruently at about 1100° C., and they do not undergo a transition phase from the melting point down to room temperature. Thus CZT single crystals can be grown by melt methods, such as the High-Pressure Bridgman (HPB), Vertical Bridgman, Horizontal Bridgman (VBHM), Traveling Heater (TH), and Zone Melting Methods (ZMM). Some difficult growth problems have been encountered in the quest to produce large uniform CZT single crystals suitable for high-efficiency, gamma-ray detectors. The most pressing problems are associated with decomposition of the melt during the crystal growth process, preferential cadmium (Cd) evaporation, and deposition of segregated Te leading to the formation of Te inclusions. These problems are ultimately limiting the performance of today's thick (>˜5 mm) commercial CZT detectors.
Tellurium inclusions are usually formed during the solidification of the crystal. When growing CZT single crystals by the typical melt methods, there is always surplus space in the upper section of the ampoule (also referred to as a reaction chamber) above the melt. In the CZT crystal growth process, cadmium evaporates from the melt and exists in the form of vapor within the upper surplus space of the growth ampoule. The evaporation and subsequent condensation of cadmium disturbs the stoichiometric composition of the CZT melt, causing the melt to become increasingly tellurium rich. Since the vapor pressure of tellurium is much lower than that of cadmium, the effects of tellurium vapor in the gas and of elemental cadmium in the melt can be ignored (i.e., for stoichiometric starting materials). This tellurium-rich melt is the ultimate cause of the Te inclusions present in CZT detectors produced by conventional commercial processes; consequently, growers must effectively control the composition of the melt to produce high-quality, inclusion-free crystals. Post-growth thermal annealing in a Cd overpressure is helpful to reduce the size of the Te inclusions in the as-grown crystals, but the residual damage caused by the Te-rich inclusions remains after thermal annealing.
While Te inclusions are found throughout the crystal volume, higher concentrations are observed near grain and twinning boundaries of crystals. FIGS. 2(a)-2(d) illustrate infrared (IR) images of Te inclusions showing the shapes of typical Te inclusions in different orientations and structures. For example, using Miller indice notation, the various images include FIG. 2(a) (112), FIG. 2(b) (111), FIG. 2(c) twin between (115) and (111), FIG. 2(d) decoration of a mini-mosaic structure. The density of CZT Te inclusions in the current CZT crystals is about 105 cm−3. They are dispersed throughout the crystals and often form meandering and straight “trails”, most likely following regions of high dislocations. Tellurium inclusions act like a short circuit path that degrades the material and disrupts the electrical signal. Te inclusions can be detected using IR transmission microscopy or high spatial resolution x-ray mapping techniques. For example, FIG. 3 illustrates the correlation between Te inclusions as identified by dark spots in an IR image and degraded zones displaying poor charge collection as seen in the X-ray map. The X-ray scan area is ˜1.2×1.2 mm2, the beam size is 10×10 μm2, and the step size is 10 μm.
In practically all cases, a traditional Bridgman growth method is used conventionally to grow CZT and CdTe detectors. The presence of inclusions was not previously recognized as an important limiting factor in crystal functionality. Only recently has it been determined that that inclusions were the major cause limiting the device performance for thick gamma ray and x-ray CZT detectors. G. A. Carini, A. E. Bolotnikov, G. S. Camarda, G. W. Wright, and R. B. James, “Effect of Te-precipitates on performance of CdZnTe detectors”, Appl Phys Lett, Vol. 88, pp. 143515, 2006. In fact, tellurium inclusions were long presumed to be benign in terms of device quality. Given the new knowledge, there is a recognized need to reduce the size and density of tellurium inclusions, or eliminate them altogether.
One possible way to eliminate the formation of Te inclusions is by increasing the stirring of the melt through accelerated-decelerated rotation of the furnace. Although doable, this is an extraordinarily difficult approach that has been modeled, but not yet attempted for CZT and CdTe crystal growth. The presence of inclusions at the surface of CZT infrared substrates seriously disrupts growth of the mercury cadmium telluride epi-layer and performance of the IR imaging arrays. Hence a simple yet elegant approach is needed to eliminate the formation of Te inclusions during crystal growth.